(1) Statement of the Invention
The present invention relates to thermally stable dioxetanes which can be triggered by chemical reagents or enzymes to generate chemiluminescence in organic solvents or in aqueous solution. A method for significantly enhancing the chemiluminescence efficiency has been discovered which involves intramolecular energy transfer to a fluorescent group which is bonded or "tethered" to the dioxetane molecule. These compounds can be used in various chemiluminescent assays including enzyme-linked immunoassays and enzyme-linked DNA probes as well as direct, chemically triggerable labels for biomolecules.
(2) Prior Art
1. Mechanisms of Luminescence
Exothermic chemical reactions release energy during the course of the reaction. In virtually all cases, this energy is in the form of vibrational excitation or heat. However, a few chemical processes generate light or chemiluminescence instead of heat. The mechanism for light production involves thermal or catalyzed decomposition of a high energy material (frequently an organic peroxide such as a 1,2-dioxetane) to produce the reaction product in a triplet or singlet electronic excited states. Fluorescence of the singlet species results in what has been termed direct chemiluminescence. The chemiluminescence quantum yield is the product of the quantum yields for singlet chemiexcitation and fluorescence. These quantities are often expressed as efficiencies where efficient (%)=.PHI..times.100. Energy transfer from the triplet or singlet product to a fluorescent acceptor can be utilized to give indirect chemiluminescence. The quantum yield for indirect chemiluminescence is the product of the quantum yields for singlet or triplet chemiexcitation, energy transfer, and fluorescence of the energy acceptor. ##STR1##
2. Dioxetane Intermediates in Bioluminescence
In 1968 McCapra proposed that 1,2-dioxetanes might be the key high-energy intermediates in various bioluminescent reactions including the firefly system. (F. McCapra, Chem. Commun., 155 (1968)). Although this species is apparently quite unstable and has not been isolated or observed spectroscopically, unambiguous evidence for its intermediacy in the reaction has been provided by oxygen-18 labeling experiments. (O. Shimomura and F. H. Johnson, Photochem. Photobiol., 30, 89 (1979). ##STR2##
3. First Synthesis of Authentic 1,2-Dioxetanes
In 1969 Kopecky and Mumford reported the first synthesis of a dioxetane (3,3,4-trimethyl-1,2-dioxetane) by the base-catalyzed cyclization of a beta-bromohydroperoxide. (K. R. Kopecky and C. Mumford, Can. J. Chem., 47, 709 (1969). As predicted by McCapra, this dioxetane did, in fact, produce chemiluminescence upon heating to 50.degree. C. with decomposition to acetone and acetaldehyde. However, this peroxide is relatively unstable and cannot be stored at room temperature (25.degree. C.) without rapid decomposition. In addition, the chemiluminescence efficiency is very low (less than 0.1%). ##STR3##
Bartlett and Schaap and Mazur and Foote independently developed an alternative and more convenient synthetic route to 1,2-dioxetanes. Photooxygenation of properly-substituted alkenes in the presence of molecular oxygen and a photosensitizing dye produces dioxetanes in high yields. (P. D. Bartlett and A. P. Schaap, J. Amer. Chem. Soc., 92, 3223 (1970) and S. Mazur and C. S. Foote, J. Amer. Chem. Soc., 92, 3225 (1970)). The mechanism of this reaction involves the photochemical generation of a metastable species known as singlet oxygen which undergoes 2+2 cycloaddition with the alkene to yield the dioxetane. Research has shown that a variety of dioxetanes can be prepared using this reation (A. P. Schaap, P. A. Burns, and K. A. Zaklika, J. Amer. Chem. Soc., 99, 1270 (1977); K. A. Zaklika, P. A. Burns, and A. P. Schaap, J. Amer. Chem. Soc., 100, 318 (1978); K. A. Zaklika, A. L. Thayer, and A. P. Schaap, J. Amer. Chem. Soc., 100, 4916 (1978); K. A. Zaklika, T. Kissel, A. L. Thayer, P. A. Burns, and A. P. Schaap, Photochem. Photobiol., 30, 35 (1979); and A. P. Schaap, A. L. Thayer, and K. Kees, Organic Photochemical Synthesis, II, 49 (1976)). During the course of this research, a polymer-bound sensitizer for photooxygenations was developed (A. P. Schaap, A. L. Thayer, E. C. Blossey, and D. C. Neckers, J. Amer. Chem. Soc., 97, 3741 (1975); and A. P. Schaap, A. L. Thayer, K. A. Zaklika, and P. C. Valenti, J. Amer. Chem. Soc., 101, 4016 (1979)). This new type of sensitizer has been patented and sold under the tradename SENSITOX.TM. (U.S. Pat. No. 4,315,998 (2/16/82); Canadian Pat. No. 1,044,639 (12/19/79)). Over fifty references have appeared in the literature reporting the use of this product. ##STR4##
4. Preparation of Stable Dioxetanes Derived from Sterically Hindered Alkenes
Wynberg discovered that photooxygenation of sterically hindered alkenes such as adamantylideneadamantane affords a very stable dioxetane (J. H. Wieringa, J. Strating, H. Wynberg, and W. Adam, Tetrahedron Lett., 169 (1972)). A collaborative study by Turro and Schaap showed that this dioxetane exhibits an activation energy for decomposition of 37 kcal/mol and a half-life at room temperature (25.degree. C.) of over 20 years (N. J. Turro, G. Schuster, H. C. Steinmetzer, G. R. Faler, and A. P. Schaap, J. Amer. Chem. Soc., 97, 7110 (1975)). In fact, this is the most stable dioxetane yet reported in the literature. Adam and Wynberg have recently suggested that functionalized adamantylideneadamantane 1,2-dioxetanes may be useful for biomedical applications (W. Adam, C. Babatsikos, and G. Cilento, Z. Naturforsch., 39b, 679 (1984); H. Wynberg, E. W. Meijer, and J. C. Hummelen, In Bioluminescence and Chemiluminescence, M. A. DeLuca and W. D. McElroy (Eds.) Academic Press, New York, p. 687, 1981; and J. C. Hummelen, T. M. Luider, and H. Wynberg, Methods in Enzymology, 133B, 531 (1986)). However, use of this extraordinarily stable peroxide for chemiluminescent labels requires detection temperatures of 150.degree. to 250.degree. C. Clearly, these conditions are unsuitable for the evaluation of biological analytes in aqueous media. McCapra, Adam, and Foote have shown that incorporation of a spirofused cyclic or polycyclic alkyl group with a dioxetane can help to stabilize dioxetanes that are relatively unstable in the absence of this sterically bulky group (F. McCapra, I. Beheshti, A. Burford, R. A. Hann, and K. A. Zaklika, J. Chem. Soc., Chem. Commun., 944 (1977); W. Adam, L. A. A. Encarnacion, and K. Zinner, Chem. Ber., 116, 839 (1983); G. G. Geller, C. S. Foote, and D. B. Pechman, Tetrahedron Lett., 673 (1983); P. Lechtken, Chem. Ber., 109, 2862 (1976); and P. D. Bartett and M. S. Ho, J. Amer. Chem. Soc., 96, 627 (1974)) ##STR5##
5. Effects of Substituents on Dioxetane Chemiluminescence.
The stability and the chemiluminescence efficiency of dioxetanes can be altered by the attachment of specific substituents to the peroxide ring (K. A. Zaklika, T. Kissel, A. L. Thayer, P. A. Burns, and A. P. Schaap, Photochem. Photobiol., 30, 35 (1979); A. P. Schaap and S. Gagnon, J. Amer. Chem. Soc., 104, 3504 (1982); A. P. Schaap, S. Gagnon, and K. A. Zaklika, Tetrahedron Lett., 2943 (1982); and R. S. Handley, A. J. Stern, and A. P. Schaap, Tetrahedron Lett., 3183 (1985)). The results with the bicyclic system shown below illustrate the profound effect of various functional groups on the properties of dioxetanes. The hydroxy-substituted dioxetane (X.dbd.OH) derived from the 2,3-diaryl-1,4-dioxene exhibits a half-life for decomposition at room temperature (25.degree. C.) of 57 hours and produces very low levels of luminescence upon heating at elevated temperatures. In contrast, however, reaction of this dioxetane with a base at -30.degree. C. affords a flash of blue light visible in a darkened room. Kinetic studies have shown that the deprotonated dioxetane (X.dbd.O.sup.-) decomposes 5.7.times.10.sup.6 times faster than the protonated form (X.dbd.OH) at 25.degree. C. ##STR6##
The differences in the properties of these two dioxetanes arise because of two competing mechanisms for decomposition ((K. A. Zaklika, T. Kisse, A. L. Thayer, P. A. Burns, and A. P. Schaap, Photochem. Photobiol., 30, 35 (1979); A. P. Schaap and S. Gagnon, J. Amer. Chem. Soc., 104, 3504 (1982); A. P. Schaap, S. Gagnon, and K. A. Zaklika, Tetrahedron Lett., 2943 (1982); and R. S. Handley, A. J. Stern, and A. P. Schaap, Tetrahedron Lett., 3183 1985)). Most dioxetanes cleave by a process that involves homolysis of the O--O bond and formation of a biradical. An alternative mechanism is available to dioxetanes bearing substituents such as O.sup.- with low oxidation potentials. The cleavage is initiated by intramolecular electron transfer from the substituent to the antibonding orbital of the peroxide bond.
6. Chemical Triggering of Stabilized 1,2-Dioxetanes
We have recently discovered that thermally stable dioxetanes can be triggered by chemical and enzymatic processes to generate chemiluminescence on demand (A. P. Schaap, patent application Ser. No. 887,139, filed July 15, 1986; A. P. Schaap, R. S. Handley, and B. P. Giri, Tetrahedron Lett., 935 (1987); A. P. Schaap, T. S. Chen, R. S. Handley, R. DeSilva, and B. P. Giri, Tetrahedron Lett., 1155 (1987); and A. P. Schaap, M. D. Sandison, and R. S. Handley, Tetrahedron Lett., 1159 (1987)). To do this, we have developed new synthetic procedures to produce dioxetanes with several key features: (1) the stabilizing influence of spiro-fused adamantyl groups has been utilized to provide dioxetanes that have "shelf lives" of years at ambient temperature and (2) new methods for triggering the chemiluminescent decomposition of the stabilized dioxetanes have been provided.
The required alkenes have been prepared by reaction of 2-adamantanone with aromatic esters or ketones using titanium trichloride/LAH in THF (A. P. Schaap, patent application Ser. No. 887,139). This is the first report of the intermolecular condensation of ketones and esters to form vinyl ethers using the McMurry procedure. Although McMurry had earlier investigated the intramolecular reaction of ketone and ester functional groups, cyclic ketones and not vinyl ethers were prepared by this method (J. E. McMurry and D. D. Miller, J. Amer. Chem. Soc., 105, 1660 (1983)). ##STR7##
Photooxygenation of these vinyl ethers affords dioxetanes that are easily handled compounds with the desired thermal stability. For example, the dioxetane shown below exhibits an activation energy of 28.4 kcal/mol and a half-life at 25.degree. C. of 3.8 years. Samples of this dioxetane in o-xylene have remained on the laboratory bench for several months with no detectable decomposition. ##STR8##
However, the chemiluminescent decomposition of this dioxetane can be conveniently triggered at room temperature by removal of the silyl-protecting with fluoride ion to generate the unstable, aryloxide form which cleaves to yield intense blue light. The half-life of the aryloxide-substituted dioxetane is 5 seconds at 25.degree. C. The spectrum of the chemiluminescence in DMSO exhibited a maximum at 470 nm which is identical to the fluorescence of the anion of the ester cleavage product (methyl 3-hydroxybenzoate) and the fluorescence of the spent dioxetane solution under these conditions. No chemiluminescence derived from adamantanone fluorescence appears to be produced. Chemiluminescence quantum yields for the fluoride-triggered decomposition measured relative to the luminol standard was determined to be 0.25 (or a chemiluminescence efficiency of 25%). Correction for the fluorescence quantum yield of the ester under these conditions (.PHI..sub.F =0.44) gave an efficiency for the formation of the singlet excited ester of 57%, the highest singlet chemiexcitation efficiency yet reported for a dioxetane prepared in the laboratory. ##STR9##
7. Enzymatic Triggering of 1,2-Dioxetances
Biological assays such as immunoassays and DNA probes involving enzymes utilize a wide variety of substrates which either form a color (chromogenic) or become fluorescent (fluorogenic) upon reaction with the enzyme. As part of our investigation of triggering methods, we developed the first dioxetanes which can function as chemiluminescent enzyme substrates (A. P. Schaap, patent application Ser. No. 887,139; A. P. Schaap, R. S. Handley, and B. P. Giri, Tetrahedron Lett., 935(1987); A. P. Schaap, T. S. Chen, R. S. Handley, R. DeSilva, and B. P. Giri, Tetrahedron Lett., 1155 (1987); and A. P. Schaap, M. D. Sandison, and R. S. Handley, Tetrahedron Lett., 1159 (1987)). Use of these peroxides in biological systems requires dioxetanes which are thermally stable at the temperature of the enzymatic reaction and do not undergo rapid spontaneous decomposition in the aqueous buffers. The spiro-fused adamantyl dioxetanes described in the previous section meet these requirements. We have, therefore, prepared dioxetanes bearing functional groups which can be enzymatically modified to generate the aryloxide form. Decomposition of this unstable intermediate provides the luminescence. Dioxetanes have been synthesized which can be triggered by various enzymes including aryl esterase, acetylcholinesterase, and alkaline phosphatase. The phosphatase example is particularly significant because this enzyme is used extensively in enzyme-linked immunoassays. ##STR10##
For example, enzymatic triggering by alkaline phosphatase was observed with the phosphate-substituted dioxetane derived from 3-hydroxy-9H-xanthen-9-one and 2-adamantanone. The dioxetane is thermally stable with an activation energy of 30.7 kcal/mol and a half-life at 25.degree. C. of 12 years. The dioxetane is not only stable in organic solvents but also shows very slow spontaneous decomposition in aqueous buffers.
Triggering experiments were conducted using alkaline phosphatase from bovine intestinal mucosa [suspension of 5.3 mg of protein (1100 units/mg protein) per mL in 3.2M (NH.sub.4).sub.2 SO.sub.4 ] and the phosphate-protected dioxetane at pH 10.3 in 0.75M 2-amino-2-methyl-1-propanol buffer. A 50 .mu.L aliquot (0.013 .mu.mol) of a phosphate-dioxetane stock solution was added to 3 mL of the buffer at 37.degree. C. to give a final dioxetane concentration of 4.2.times.10.sup.-6 M. Injection of 1 .mu.L (final concentration of protein=1.8 .mu.g/mL) of alkaline phosphatase to the solution resulted in burst of chemiluminescence that decayed over a period of 3 minutes. Over this period of time, the background luminescence from slow non-enzymatic hydrolysis of the dioxetane in the buffer was only 0.2% of that produced by the enzymatic process. The total light emission was found to be linearly dependent on the dioxetane concentration. The rate of decay of the emission is a function of enzyme concentration while the total light emission is independent of the enzyme concentration. The chemiluminescence spectrum for the phosphatase-catalyzed decomposition was obtained at room temperature in the buffer solution. A comparison of this chemiluminescence spectrum with the fluorescence spectrum of the spent reaction mixture and the fluorescence spectrum of the hydroxyxanthanone cleavage product in the buffer indicates that the emission is initiated by the enzymatic cleavage of the phosphate group in dioxetane to yield the unstable aryloxide dioxetane which generates the singlet excited anion of hydroxyxanthanone.